This invention relates to cathode ray tube scanning systems and more particularly to apparatus for correcting scanning raster distortions known as top-bottom pin-cushion distortions.
As is well known in the cathode ray tube scanning art, the display raster of a cathode ray tube is composed of successive spaced horizontal scan lines to form a substantially rectangular raster. Although the raster is desirably rectangular under normal operating conditions, it is also well known that the raster is frequently distorted at the top and bottom in what is known as top-bottom pin-cushion distortion.
As is also well known, top-bottom pin-cushion distortion is indicated by a departure of the horizontal scanning lines from a straight line. Specifically, the horizontal scan lines at the top half of the raster tend to bow downwardly toward the center of the raster while the horizontal scan lines at the bottom half of the raster tend to bow upwardly toward the center of the raster.
One known form of apparatus for correcting undesired top-bottom pin-cushion distortion utilizes a one-quarter inch E core saturable reactor wherein the control winding of the pin-cushion correction transformer is in series connection with the horizontal deflection winding of the yoke. Thus, the relatively high horizontal yoke current is routed through the pin-cushion transformer winding which may have an adverse effect upon such features as linearity and S-curve correction. Moreover, it has been found that such apparatus may be satisfactory for cathode ray tubes having a deflection angle up to about 90.degree. but tends to result in center compression and inadequate pin-cushion correction when cathode ray tubes with 110.degree. deflection angles are employed.
In another known form of apparatus for correcting top-bottom pin-cushion distortion, a one-quarter inch E-core saturable reactor is utilized wherein a control winding is coupled to a separate winding on the flyback transformer. However, when the flyback pulse signal ends or returns to zero the control winding of the pin-cushion apparatus is, for all practical purposes, short-circuited. Thus, there is no ringing signal for transfer to the secondary winding of the pin-cushion correction apparatus.